Re: Article 20 of the German Constitution strictly forbids ...
No, that is not right. It is not about the actual enforcement, it is about the decision-making. So if a patent-office would make a decision that some patent application is OK then any offence would be a crime, possibly leading to punishment of German citizens or inhabitants. What I said is that such a decision must be done by a gremium with at least one officially appointed German official and that such gremiums must make all their decisions unanimously; but this is probably not the case. For this reason such an organisation would be illegal by German law (and in fact probably by the laws/constituions of most countries in the world).
International treaties are partly a different topic, because here you in most cases you simply have the treaty which is fixed; there is no organisation which makes any decisions later on. Please also note that a treaty by definition is agreed upon unanimously; this is part of the definition of a treaty.
The implication of Article 20 however, is far greater: It explictily forbids most of the EU organisations incl. the EU commission, the EU (fake-)parliarment, most the EU administrations etc. This is the real importance of Article 20.